In recent years, a vehicle includes a keyless entry device in which, a driver holds an electronic key including a transmitting function brings her/his hand close to or into contact with a door handle to automatically unlock or lock the door. The keyless entry device includes a proximity sensor built in a door handle of a vehicle, an antenna that transmits and receives a signal to/from an electronic key when the proximity sensor detects proximity or contact of a hand of person to the door handle, an actuator that locks or unlocks the door of the vehicle, and a controller that detect an ON/OFF state of the proximity sensor with an output from the proximity sensor and drives the actuator to lock or unlock the door when a code signal from the electronic key received by the antenna when the proximity sensor is turned on is matched with a code signal inherent in the vehicle.
A proximity sensor used in such a keyless entry device detects proximity or contact of a hand of person with an electrostatic capacity system. This is proposed in, for example, Patent Documents 1 and 2 or the like.
FIG. 16 shows FIG. 4 in Patent Document 1. A proximity sensor described in Patent Document 1 will be described below with reference to FIG. 4. The proximity sensor includes a circuit board on which a detection circuit is printed and a drive electrode (reference electrode) and a detection electrode (measuring electrode) which are mounted on the circuit board. In this case, the drive electrode and the detection electrode are surface-mounted on the circuit board, and are electrically connected to the detection circuit.
When a pulse-like voltage is applied to the drive electrode of the proximity sensor, an electric field is generated between the drive electrode and the detection electrode. When a hand of person is brought close to or into contact with the proximity sensor, electric flux lines emitted from the drive electrode to the detection electrode are partially absorbed with the hand of person serving as a dielectric substance. At this time, the electric flux lines reaching the detection electrode decrease in number. For this reason, a voltage induced by the detection electrode decreases. More specifically, when the decrease in voltage is measured to make it possible to detect that the hand of person is brought close to or into contact with the proximity sensor.
As shown in FIG. 4B in Patent Document 3, a proximity sensor in which a detection electrode of an unlock sensor is disposed on one mounting surface of a circuit board on which a detection circuit is printed and a detection electrode of a lock sensor is disposed on the other mounting surface is also disclosed.